This proposal seeks funds to study the structure/function aspects of human IgA. Basic information is being sought about this immunoglobulin. The overall goals of this application are two-fold: (1) to define the molecular mechanisms of IgA function in mucosal immunity and (2) to exploit this basic knowledge to produce potentially therapeutic molecules and target these molecules to mucosal epithelial cells. Much of what will be learned has direct clinical relevance as IgA is a crucial mediator of mucosal immunity, and there is a clear need for novel approaches to protecting humans as well as animals from foreign invaders that penetrate the mucosal barrier. However, even without the clinical relevance, this proposal is important in that basic immunological and cell biological information will derive from an understanding of the interaction between IgA and the polymeric immunoglobulin receptor and other effector modalities. The proposal aims to use recombinant DNA techniques and phage display technology to identify the regions of IgA which interact with the polymeric immunoglobulin receptor, streptococcal IgA-binding proteins and complement proteins. Three important preliminary findings provide confidence that the major aims of the proposal can be achieved. First, the domain of human IgA that interacts with the polymeric immunoglobulin receptor has been identified. Second, a peptide has been selected by phage display that further localizes the site of interaction. Crucial is the fact that the IgG molecule does not even bear this "loop" in its tertiary structure. Third, it has been demonstrated that dimeric IgA administered intravenously in a macaque model can be detected on the mucosal surface suggesting that our goal of using a small peptide to serve as a vehicle to target variable regions of antibody or even drugs or genes to epithelial cells and/or mucosal surfaces is realistic. Identification of a small polymeric immunoglobulin receptor-binding peptide will be approached by independent techniques thus allowing selection of a high affinity interaction. Furthermore, the definition of the bacterial protein binding sites of IgA will aid in the design of streptococcal-lgA agonists. The complement analysis will definitively settle the long-standing controversy regarding the complement activating potential of IgA as well as provide valuable information for the rational design of mucosal-directed therapeutic molecules. Finally, the x-ray crystal structure will be obtained by employing a method to over-express IgA Fc molecules using leucine zipper sequence to facilitate pairing. This information will aid in defining the molecular mechanisms of IgA function and the overall development of IgA-derived therapeutics.